1. Field of the Invention
The present invention relates to a thermal displacement correction device for a machine tool and, more particularly, to a method of calculating a correction for thermal displacement of a spindle unit.
2. Description of the Related Art
A feed screw and a spindle of a machine tool are driven by motors so that heat generation of the motors, frictional heat generated by the rotation of bearings, and frictional heat of a coupling portion of a ball screw and a ball nut constituting the feed screw cause the feed screw and the spindle to expand and mechanical positions are displaced. That is, the relative position of a workpiece to be positioned and a tool is displaced. Variations in mechanical position due to heat are problematic in the case of high-precision machining.
Methods of estimating a displacement from various elements in order to remove the displacement are well known. JP 2003-108206 A discloses a correction device that establishes a three-dimensional model of a machine and estimates thermal displacement corrections from the operation and temperature of the machine. This correction device estimates a change in the temperature of micro-regions and estimates a thermal displacement correction from the temperature change estimated, thereby enabling an accurate correction. JP 2006-281420 A discloses a correction method for a NC machine tool that calculates a temperature distribution function based on temperature measurements at multiple points of a column and thereby calculates thermal displacement of a spindle unit.
The correction device described in JP 2003-108206 A is computationally complex due to a lot of computations so that it is not suitable to estimate the thermal displacement corrections varying from moment to moment. The correction method described in JP 2006-281420 A, which uses temperature sensors, increases the cost of a machine tool and can no longer perform accurate correction in failure of the temperature sensor.